The following studies will be performed to approach an understanding of the mechanism by which Simian Virus 40 (SV40) produces oncogenic transformation of mammalian cells: 1) Balb 3T3 mouse cells will be transformed by several tsA mutants of SV40 and the resulting tsA transformants will be studied to investigate the role of continued expression of the SV40 A gene in the maintenance of transformation. 2) The role of the A gene product in producing alterations in eukaryotic DNA synthesis in transformed cells will be studied by assaying net host DNA synthesis, by applying electron microscopic techniques to the analysis of eukaryotic DNA replication intiation and by studying the temporal sequence of eukaryotic DNA replication. 3) Regulation of SV40 A gene expression will be studied by investigating the effects of gene dosage and gene location (i.e., the SV40 integration site).